Doesn't constraining the "auto" in C++ defeat the purpose of it?

Solution 1:

A constraint on the deduced auto type doesn't mean it needs to be a specific type, it means it needs to be one of a set of types that satisfy the constraint. Note that a constraint and a type are not the same thing, and they're not interchangeable.

e.g. a concept like std::integral constrains the deduced type to be an integral type, such as int or long, but not float, or std::string.

If I really need a std::integral datatype, couldn't I just omit the auto completely?

In principle, I suppose you could, but this would at the minimum lead to parsing difficulties. e.g. in a declaration like

foo f = // ...

is foo a type, or a constraint on the type?

Whereas in the current syntax, we have

foo auto f = // ...

and there's no doubt that foo is a constraint on the type of f.

Solution 2:

If I really need a std::integral datatype, couldn't I just omit the auto completely?

No, because std::integral is not a type, it's a concept, a constraint on types (or if you will, a set of types rather than a single type).

Doesn't this defeat the purpose of auto here?

The original purpose of auto in C++11 is telling the compiler: Whatever type you deduce.^*

With C++20, auto has an expanded use case - together with a concept, a constraint over types. auto still tells the compiler: Whatever type you deduce - but the deduction must also respect the constraint.

_{* - ignoring issues like constness, l/rvalue reference etc.}

Solution 3:

A concept often just move the error earlier in the compilation and makes code a bit more readable (since the concept name is a hint to the reader what you require from a type).

Rephrased:

It is rare you will ever use an auto variable in a way that it will work on every type.

For example:

auto fn(auto x) {
    return x++;
}

will not work if you do:

f(std::string("hello"));

because you can not increment std::string, the error is something like:

error: cannot increment value of type 'std::basic_string<char>'
    return x++;

If you change the function to:

auto fn(std::integral auto x) {
    return x++;
}

You will get an error like:

:6:6: note: candidate template ignored: constraints not satisfied [with x:auto = std::basic_string] auto fn(std::integral auto x) {

For a small example this, it does not matter a lot, but for real code often the fn would call fn2 that calls fn3... and you would get the error deep in the std/boost/... implementation file.

So in this way concepts move the error to the site of the first function call.